1. Field of the Invention
This invention relates to an idling rotational speed control system for internal combustion engines, and more particularly to an idling rotational speed control system which has an abnormality-determining function.
2. Description of the Related Art
There is conventionally known an idling rotational speed control system for internal combustion engines, for example, in Japanese Laid-Open Patent Publication (Kokai) No. 64-77729, which carries out feedback control of the engine rotational speed to a low desired engine rotational speed in order to prevent engine stalling, when the engine rotational speed is decelerated or idling.
Further, there is also conventionally known a method of abnormality determination, for example, from Japanese Laid-Open Patent Publication (Kokai) No. 4-43841, which determines that an idling rotational speed control system is abnormal when the engine rotational speed falls outside a predetermined range including a desired engine rotational speed as a central value thereof during feedback control of the engine rotational speed and at the same time a control parameter used in the feedback control (e.g. a control current value applied to an electromagnetic valve for controlling an amount of auxiliary air) falls outside a predetermined range.
When the driver of an automotive vehicle in which the engine is installed carries out so-called "pumping" of the brake, i.e., repeatedly applies the brakes (the brake system is repeatedly alternately operated and stopped), however, the engine rotational speed can increase depending on the relationship between the capacity of a brake master cylinder of the brake system and the volume of the intake pipe of the engine. As a result, if the afore-mentioned conventional abnormality determination method is employed, there is a fear that it is erroneously determined that the idling rotational speed control system is abnormal although it is actually functioning normally.
Further, when the air-fuel ratio of an air-fuel mixture supplied to the engine becomes overly rich or overly lean, the engine rotational speed feedback control has degraded convergency of the engine rotational speed to the desired engine rotational speed, which can result in an erroneous determination that the idling rotational speed control system is abnormal although it is actually functioning normally.
Still further, in a manual transmission type vehicle, the vehicle is sometimes driven with a clutch thereof in a half-engaged state and an accelerator pedal of the vehicle not stepped on during traveling at an extremely low speed, on such occasions as road congestion or waiting for a signal light. In such a half-engaged state of the clutch, the engine rotational speed decreases, and then the idling rotational speed control system operates to increase the engine rotational speed. However, then the idling rotational speed control system can be erroneously determined to be abnormal, depending on the feedback control response lag, load applied on the engine due to the half-engaged state of the clutch, and the duration of the half-engaged state, though it is actually functioning normally.